Two types of known control means enabling simultaneous control in more than a single degree of freedom are a joystick and a mouse of the type used for controlling computers. However, these control means have several drawbacks.
A mouse thus permits simultaneous control in only two degrees of freedom.
A joystick has also normally only two degrees of freedom. However, such a stick can be designed for control in three or possibly even more degrees of freedom. However, when using a joystick with more than two degrees of freedom it is difficult or impossible to achieve good accuracy of control and it is also difficult for the operator to coordinate movements of the joystick with movements of the controlled object.
If control is required in more degrees of freedom than two (or possibly three), as is often the case, the mouse or joystick can, in a manner known per se, be switched between control of different sets of degrees of freedom. For instance, when programming an industrial robot it is known to use a joystick with three degrees of freedom where the function of the joystick can be switched to control either the position of the tool center point of the robot hand (three translation variables) or the orientation of the robot hand (three rotation variables). However, this makes the control complicated and slow.
Furthermore, these known control means have mechanical limitations which reduce their flexibility and thus their general usefulness. A mouse, for instance, requires a flat surface such as a table or the like, and a joystick must be mounted and journalled on some form of base or platform to which the position of the stick is referred. This means that these control means are typically cumbersome and often require the operator to use both hands. They are therefore unsuitable or unusable in many applications such as in uncomfortable work positions and/or in confined spaces.
Another considerable drawback of these known control means is that, when more than two degrees of freedom are involved it is difficult or impossible to obtain a natural agreement in all the controlled degrees of freedom between movements of the control means and movements of the controlled object. The work of the operator therefore becomes complicated and slow.
A known control system enabling control in six degrees of freedom is the "Polhemus system" (Polhemus Inc., Colchester, Vt., USA). The system uses a triple-axis magnetic dipole source and a triple-axis magnetic field sensor. By sequentially varying the magnetic field emitted, three mutually independent excitation vectors are produced. The three vectors sensed by the sensor contain sufficient information to determine the position and orientation of the sensor in relation to the source. However, this system has a number of drawbacks, one of which is its sensitivity to other magnetic fields in the vicinity, which e.g. may hinder its use in workshop environments, where there are a number of varying magnetic fields from motors, etc. Another drawback is that large metal objects in the vicinity have a negative effect on the accuracy of the system which in practice makes it unusable on the production line of automobile bodies, for instance. Another drawback is that the sensor must be relatively close to the source of the magnetic field, which greatly limits its work area if high accuracy is also required. These drawbacks mean that the system can only be used in special environments.
A control means for a computer is known through patent application WO 9007762 A1. A pen-shaped control member contains a transmitter that emits a modulated signal, e.g. an optical or acoustic signal. The control member can be moved in a plane in which three fixed receivers are arranged. The phase differences between the signals received are determined and produce the differences in the distance between the control member (transmitter) and the receivers, and thereby the position of the control member in the plane. By arranging a fourth receiver outside said plane the position of the control member can be determined in three dimensions. It is implied that the orientation of the control member can also be determined by providing the control member with more transmitters.
A similar three-dimensional control means is known through patent application EP 0420500 A2. A pen-shaped control member is provided with two acoustic transmitters spaced from each other. The positions of the transmitters in three dimensions can be determined by measuring the transmission times from the transmitters to each of four fixed receivers. It is also mentioned that the measured positions of the transmitters can be used to determine the orientation of the control member in two degrees of freedom. Arranging three transmitters on the control member would also allow determination of the orientation in the third degree of freedom.
Yet another control means of similar type--a three-dimensional mouse for controlling a computer cursor--is known through EP 0526015 A1. The mouse has an acoustic transmitter and three receivers are arranged around the computer screen. The transmission times of the audio signals to the three receivers are determined and the position of the transmitter (mouse) is calculated in three dimensions from this information.
In the case of the three last control means discussed, the positions of the transmitters are determined in relation to fixed receivers. In two of these arrangements the control member is provided with two or more transmitters located a certain distance apart, and the orientation of the control member is then calculated on the basis of the measured positions of the transmitters in relation to the fixed receivers and of the distance between the transmitters. To be of any practical use, a hand-held control member of this type must have very limited dimensions, e.g. at the most of the order of magnitude of a decimetre or so. The short distances between the different transmitters in the control member, together with the unavoidable inaccuracy and the limited resolution of the determination of the positions, means that the accuracy and the resolution in the determination of the orientation is low. The accuracy and the resolution in the determination of the orientation may possibly be sufficient for the stated area of application--control of a computer display--but in practice they are certainly not sufficient for more demanding control tasks, e.g. for controlling an industrial robot or other machine that must perform with precision.
A basic feature of these three control means is that each requires at least three fixed receivers receiving signals from the transmitter (or transmitters) in the control member, in order to determine its position. These receivers are relatively complicated and expensive and they must be connected by means of cables or other signal channels to some form of common signal-processing equipment. Furthermore the work areas with regard to position and orientation of the control member of the devices described are extremely limited. Practical use in arbitrary environment usually requires a large work area and that, throughout its entire work area, the device is able to function even if parts of the field of vision of the transmitters are blocked by the operator or by machine parts or work pieces. For this to be possible, the devices described would have to be provided with a large number of receivers distributed over the work area, which would make them expensive and complicated.